The present invention relates to a two-stage positive displacement pump for liquefied petroleum gases ("LPG") in liquid phase and particularly to a fuel injection process featuring such a pump for motor vehicle engines. One of the aims of the invention is to design such a pump of low power that can be used for the feeding of liquid fuel to motor vehicle engines running on LPG, suitable, for example, for private vehicles or small utility vehicles. To be suitable for use in motor vehicles, the pump must exhibit the features of compactness, low manufacturing cost and low power consumption (particularly if electrically powered).
The engineering techniques currently employed for high power LPG pumps (several m.sup.3 /hour) are difficult to transpose to the designing of low power pumps (several liters or tens of liters per hour). Of the two types of pump used, centrifugal pumps or gear pumps, neither lends itself to minaturization sufficient to permit its use in motor vehicles: the difficulties inherent in the use of liquid LPG (elevated pressure at pump outlet, delubricating effect, solvency, low viscosity) would make the design of miniature centrifugal pumps difficult and expensive.
Besides, existing small-size gear pumps cannot withstand the delubricating effect of LPG, unless the gears are adjusted somewhat slackly, which leads to over-dimensioning and to a high rate of leakage, or unless special materials are used and very fine machining is performed, which leads to prohibitive manufacturing costs.
Another difficulty encountered in the pumping of liquid LPG derives from the fact that the liquid stored in the tank is in equilibrium with its own saturated vapor. Any drop in pressure--for example, due to leakage or to suction--causes bubbles of gas to appear within the liquid. For this reason, it is impossible to use conventional piston or diaphragm pumps for the pumping: during the suction phase, vaporization of the LPG takes place and the compression phase mainly serves to reliquefy the vaporized volume; efficiency is therefore very low and in particular cavitation will occur, leading to rapid deterioration of the pump components.